Genetic and environmental influences on separation anxiety disorder symptoms and their moderation by age and sex

We estimated genetic and environmental influences on mother-rated DSM-III-R separation anxiety disorder (SAD) symptoms in 2043 3 to 18-year-old male and female twin pairs and their siblings (348 pairs) recruited from the Australian NH&MRC Twin Registry. Using DeFries and Fulker's (1985) multiple regression analysis, we found that genetic and shared environmental influences both contributed appreciably to variation in SAD symptoms (h² = .47, SE = .07; c² = .21, SE = .05) and were significantly moderated by both sex and age. Genetic influences were greater for girls than boys (h² = .50 and .14, respectively), whereas shared environmental influences were greater for boys than girls (c² = .51 and .21, respectively). Genetic influences increased with age, whereas shared environmental influences decreased with age. Shared environmental influences were greater in magnitude for twins than for nontwin siblings (c² = .28 versus. 13, respectively). Implications of these findings for theories of the cause of separation anxiety are discussed.